Shell completion

[JoshOldenburg]

I recently created grunt-init-completion to add shell completion for grunt-init. See https://npmjs.org/package/grunt-init-completion.

npm install --global grunt-init-completion
grunt-init-completion completion --save

I decided to do this with an external module because shell completion is confusing enough as is and I thought this would be simpler, but I am willing to have it added to the core.